Transition

You will get a Colossus out as soon as you possibly can and will not stop producing them from your Robotics Facility for the rest of the game - unless you need to build a replacement Observer. You should be able to get an expansion off safely anywhere between 40 and 50 supply - judge what the Terran is doing with your Observer before you commit to an expansion. 3+ Colossus will keep you safe from any amount of Marine/Marauder attacks (before higher tech -Medivac/Viking/Banshee- at the very least).

If the Terran has Starports, do not cancel your Robotics Bay since you will likely still want to build Colossi later, instead build one Stargate, plus an additional Stargate for each Starport with a Reactor attachment that Terran has. A Starport with a Reactor attachment will be building two Vikings at a time, while a Starport with a Tech Lab will be building Banshees. If you scout both a Starport with a reactor and a bio build (2-3 Barracks) your opponent is going MMM and it's still viable to make Colossi. If the Terran already has more Vikings than you have Phoenix, be sure to Chronoboost your Stargates. At the same time, you wish to be producing Stalkers and Sentries as well. Phoenixes counter Banshees while Stalkers counter Vikings. Sentries provide excellent support.

If the Terran has taken an expansion, delay your first Colossus and get an expansion yourself. You will not be behind, Protoss tend to get their expansions slightly slower than Terran. You will also likely be able to pressure the Terran with Colossus/ground units as well.